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Quick question. I have my ab mold almost ready to cast (except for the walls and some smoothing) and was wondering about doing a mother mold for it. I read in Thurston's book that you can make a ''Mother mold'' for silicone molds. I take it as, pour on the silicone (thin stream) wait for this to cure, then go ahead with plaster or ultra-cal on top of that to save on silicone.

I use Liquid Latex and then pour plaster over the back of it to keep the shape. You then take off the latex from your sculpt, place it back into the "plaster matrix" or backing and then pour whatever you want into the latex. IE: plaster or whatever your using for your mold. Quick and easy

That would probable work as well, .60 would get alot of detail, just made sure you lay the fiberglass fairly thick so that it doesnt heat the plastic too much when you pour resin or whatever your going to be using for your master mold.

There are two basic ways to save silicone. One is to brush on a layer of thickened silicone and then make a mother mold over that. (Amateurs usually use reinforced "plaster" some sort.)

The other is to pre-make a matching mother mold, which sits some distance from the surface you're casting, and pour liquid silicone in between.

A standard way to do that is to make a bazillion little balls of clay (some kind that doesn't inhibit the cure of the silicone you're using), and cover the object you're molding with them. Then mash and smooth them into a sheet. So if you want a fairly uniform 1/4" layer of silicone, put on a bunch of roughly 3/8" balls and then smooth them out.

(Another similar thing is to cover most of the object in strips of clay cut from a sheet extruded with a pasta machine, then cut wedges to fill in the gaps, then smooth it over.)

Once you've got the desired thickness of clay, cast your mother mold over it. (Be sure to "key" it---there should be protrusions and indentations here and there, so that the resulting silicone mold will fit exactly where it should in the mother mold, and only where it should. You don't want it slipping and scrunching.)

Once the mother mold has set, take it off and scrape the clay off it and the object. Put the object on some kind of spacer, sticking into the mother mold the right amount, and pour silicone in between.

If you plan to make a good number of pulls out of this mold I have had great luck with mold max 30. It is a weight ratio TEN cure silicone that is a 100a :10b mix ratio. It is easy to mix and will last for a LONG time. Very little shrinkage.

Its not that expensive considering the quality and longevity. It was $85ish for a gallon kit. Smooth-On sells this as well.

The OOMO is great but rots after a few months and you will only get around 20-30 good pulls before it starts to fall apart.

I'd use firbre glass for any jacket on a silicone mold. It's strong Sturdy and light. Plaster bandages are good too if you don't want more than a couple of pulls for what you're making. Gypsum or a strong alpha plaster are pretty good for jackets too, I know a lot of FX techs and artisits swear by plasti paste too, you can get it via smooth on.
What clay are you using, depending on whether the clay has sulphur content you may find you get inhibition problems from the sulphur, I use Chavant NSP predominently when sculpting as the lack of sulphur prevetns it. Though I've had probs recently with a batch that was contaminated with it and had to re-sculpt a few plat sil gel10 appliances.
The one thing I'd suggest is to make a waste mold of your armour piece first <a waste mold is one that will be only ever used for one casting or even be destroyed during de-molding> The reason I suggest this is you can get a good master from that mold and then do all your final sanding and smoothing and whaterver filling is required to make a really good piece. This final master is what you'll use to make your production mold from. The good thing with this is you have a master to make more molds from one you're old one dies a death after multiple pulls. It may sound like a lot of trouble to go to but if you want a really good piece the time isn't a waste and the finished product will look all the better for it.

Well,
I have made some progress on this. What I did was sculpt out the ab plate, covered it in alumilite silicone, then on top of that I poured plaster of paris.
After the plaster dried, I knocked off the mold box walls. Then just seperated the pieces and put the silicone mold in the plaster. Now I will reuse the clay and start in on the chest plates.

EDIT: I think I misunderstood what Asok was suggesting, so the following is probably the wrong response to that. Worse, I responded to something in the middle of the thread, thinking it was something new... sorry (but the video below is cool).

If you just make a box and pour silicone into it, you'll mostly waste the silicone. For big stuff, that's horrendously expensive.

You only need a healthy layer of silicone around the plug, and mother mold around that to support it. You can brush on brushable (or thickened silicone) and make a mother mold over it, or cast a matching mother mold that leaves space around the plug, and pour silicone in between.

To cast the matching mold, you can put a layer sulfur-free modeling clay or wax over the mold, and cast a mother mold over that.

Here's a video on "Surface Casting" that shows that basic process. Surface casting is different from what we're talking about, and they're doing a negative mold, but the principle for making matching molds is the same.

You don't need the thickness to be precise the way they're doing it in the video, being careful to get it exactly 1/8" everywhere. You just need to avoid filling lots of space with expensive goo. (You also shouldn't cover the positive up and lock it in. For what we're talking about, it's probably better to have the male plug right side up, and pour the goo in through a vent hole in the top.)

By the way, to make a thin runny silicone suitable for brushing on, you can add a "thixotropic" additive. Places with a good selection of molding and casting compounds will have them for various silicones.